The production of acetic acid by microorganisms is well known to those familiar with the fermentation art. This acetic acid is produced by the microorganisms in dilute aqueous solutions so that its recovery in pure form involves separation from a large quantity of water. The expense of such separation has been so great that the production of acetic acid by fermentation has not been able to compete with the production of acetic acid based on petroleum fossil fuel sources. However, the gradual depletion of petroleum fossil fuel with the resultant increase in prices of petrochemical feedstocks has revived interest in such fermentation reactions which can convert carbohydrates that are renewable raw materials into acetic acid and other simple organic chemicals.
When acetic acid is made by fermentation, the acid formed soon lowers the pH of the medium to a point at which the microorganism no longer grows, and acid production eventually stops. For this reason, it is necessary to add a reagent to the fermentation reaction which will neutralize at least a part of the acetic acid and maintain the pH at a high enough level to permit continued growth of the microbe. The solution must then be acidified before the acetic acid is extracted. The neutralization and acidification steps add to the cost of the process and produce salts which must be disposed of.
It would be of considerable economic importance if a low-cost process could be developed for the extraction of acetic acid produced by such fermentation reactions. It would be an added benefit if the reagent needed to adjust the pH in the fermentation reaction could be recovered for reuse in the process.
Daniel, et al., in British Pat. No. 1,426,018, published Feb. 25, 1976, disclose a process for the recovery of an acid from an aqueous solution. The extractant is a mixture of an amine which contains at least 20 carbon atoms per molecule and a water-immiscible organic solvent. In order to obtain the acid, it is necessary to back-extract the acid from the organic solvent with water. The resultant solution still contains a large percentage of water and only a partial concentration of the acid is accomplished by this procedure.
A recent review of the methods of extraction of acetic acid from water is given by C. J. King in the proceedings of the International Solvent Extraction Conference, Series 2, Paper 80-66 (1980). This review includes a summary of the following three references which report work carried out in King's laboratory:
Wardell, et al., J. Chem. Eng. Data, 23, 144 (1978). PA1 Ricker, et al., J. Separ. Proc. Technol., 1 (1) 36-41 (1979). PA1 Ricker, et al., J. Separ. Proc. Technol., 1 (2) 23-33 (1980).
King discloses the extraction of acetic acid from dilute aqueous solutions using various solvent systems. The best solvent systems were mixtures of a high molecular weight tertiary amine or a trialkyl phosphine oxide diluted with a polar solvent. The preferred solvent was a mixture of a commercial amine, consisting largely of trioctylamine, and diisopropyl ketone. The use of tributylamine was dismissed because the tributylamine-acetic acid complex was too soluble in water for this amine to be satisfactory for the extraction.
A convenient method for the extraction of acetic acid has now been discovered. This process does not require prior acidification of the fermentation broth and permits the use of solvents previously considered to be unsuitable for the extraction. Furthermore, the reagent needed for maintaining the pH in the fermentation broth is recovered for reuse in the process.